1. Field of the Invention
This invention is related to a digital watermark embedding method for embedding a digital watermark in order to restrict copying contents from digital data.
2. Description of the Related Art
With the realization of digital television broadcasting, distribution of video images using a digital signal has become commonplace. Also, video recorders that record video images as digital data onto an optical disk or magnetic tape have become widespread. By making it possible to record video images as digital data, it becomes possible to make copies without deteriorating the video image. Therefore, from the aspect of copyrights, it is necessary to restrict the copying of video images.
The digital watermark technique disclosed in EP1006722A2 (corresponding to Japanese Patent Application Laid-open 2000-173175) is one technique of restricting copying of video images, and is a technique of embedding data in the digital video image for restricting copying. Hereafter, the data for restricting copying that is embedded in the digital image is called a digital watermark. A digital watermark is embedded such that it is hidden in the video. Therefore, there is none or very little decrease in quality of the video even with the digital watermark embedded in the video. Also, a person who receives a video having an embedded digital watermark can probably not tell that there is a digital watermark in that video, so it is extremely difficult to remove the digital watermark from the video.
In EP1006722A2, a system, which uses a digital watermark that indicates that copying is prohibited and a digital watermark that indicates that first-generation copying is allowed, is disclosed. With this system, it is possible to manage the number of times copying is allowed, so copyright problems do not occur, and it is possible for the viewer of a television broadcast to view a transmitted video without putting constraints on the broadcast time.
In this system, when the digital video is input to the video recorder, the video recorder determines whether or not there is a digital watermark in the video. When there is a digital watermark and that digital watermark indicates that copying is prohibited, the video recorder will not record that video. Moreover, when there is no digital watermark, or when there is a digital watermark and that digital watermark indicates that first-generation copying is allowed, the video recorder determines that it is allowed to record that video and records the video on an optical disk or the like.
In the case of broadcasting a video from a broadcast station, normally, a plurality of video contents are broadcast continuously. Therefore, control data for copying the video, or in other words, data such as data prohibiting copying, data allowing first-generation copying or data allowing copying, are broadcast continuously together with differing video contents. For example, it is possible that video contents, for which copying is allowed, will be broadcast after broadcasting video contents, for which copying is prohibited.
However, in order for the video recorder to detect whether or not there is a digital watermark requires a certain amount of time (for example 1 to 30 seconds). Therefore, it is not possible for detection to quickly follow the switching of video contents, and so for the beginning part of the following video contents, control data that differ from the actual control data are identified by the video recorder. For example, even though the video contents are switched from allowing copying to prohibiting copying, there is a problem that the beginning part of the following video content is copied. Moreover, in the case where the video contents are switched from allowing copying to allowing first-generation copying, normally, since a digital watermark indicating that copying is finished is further embedded in the video contents in which first-generation copying is allowed and the video contents are scrambled and recorded, there is a problem that the digital watermark indicating that copying is finished is not embedded at the beginning of the following video contents, so the contents are not scrambled.